1. Technical Field
The present invention relates to an optical article to be used as a plastic lens for spectacles, cameras, and the like, and a process for producing an optical article.
2. Related Art
Plastic lenses are lightweight and excellent in moldability, processability, dyeability, etc. and less likely to crack and have high safety as compared with glass lenses. Therefore, their use has rapidly spread in the field of lenses for spectacles and they make up a large portion thereof. Further, recently, in order to meet an additional demand of reducing the thickness and weight, a material having a high refractive index such as a thiourethane resin or an episulfide resin has been developed. For example, a process for producing an episulfide resin having a very high refractive index by polymerization of a compound having an epithio group (an episulfide compound) in the presence of sulfur has been proposed (see JP-A-2004-002712 and JP-A-2005-281527). Such an episulfide resin can easily exhibit a high refractive index of 1.7 or higher and is effective in reducing the thickness of lenses for spectacles.
On the other hand, when the refractive index of a lens substrate is increased, a primer layer and a hard coat layer should have a refractive index equal to that of the lens substrate for preventing the occurrence of interference fringe. For example, in general, any of various metal oxides is incorporated in the hard coat layer as a filler thereby increasing the refractive index. As the metal oxide, fine particles of a simple substance such as titanium oxide, zirconium oxide, antimony oxide or tin oxide or composite fine particles thereof are used. However, such a metal oxide is generally poor in light resistance and tends to be colored. Therefore, from the viewpoint of transparency and stability in the visible light range and the like, titanium oxide is mainly used. However, titanium oxide has a property of exhibiting a photocatalytic action under ultraviolet radiation. Therefore, when titanium oxide is used as a filler in the hard coat layer, it decomposes a binder component composed of an organic resin therearound, and the coat layer is often peeled off. As one of the measures for this problem, it is often the case that not anatase-type titanium oxide which is likely to exhibit a photocatalytic action, but rutile-type titanium oxide which exhibits a relatively less photocatalytic action is adopted as titanium oxide (for example, see JP-A-2007-102096).
However, such rutile-type titanium oxide do not completely have a photocatalytic action, and an attempt that the photocatalytic action is reduced by coating the surface thereof with an insulating material such as silicon dioxide has also been made, but is not sufficient. Accordingly, when light resistance is considered, a method of increasing the content of rutile-type titanium oxide in the coating layer could not simply increase the refractive index of a hard coat layer or a primer layer, and when a lens substrate has a refractive index of 1.7 or higher, the occurrence of interference fringe could not completely be prevented. Further, a problem has also arisen that the impact resistance is decreased as the thickness of a lens substrate is reduced.